Abstract

Core formation should have stripped the terrestrial, lunar, and martian mantles
of highly siderophile elements (HSEs). Instead, each world has disparate, yet
elevated HSE abundances. Late accretion may offer a solution, provided that
≥0.5% Earth masses of broadly chondritic planetesimals reach
Earth’s mantle and that ~10 and ~1200 times less mass goes to Mars and
the Moon, respectively. We show that leftover planetesimal populations dominated
by massive projectiles can explain these additions, with our inferred size
distribution matching those derived from the inner asteroid belt, ancient
martian impact basins, and planetary accretion models. The largest late
terrestrial impactors, at 2500 to 3000 kilometers in diameter, potentially
modified Earth’s obliquity by ~10°, whereas those for the Moon, at
~250 to 300 kilometers, may have delivered water to its mantle.